Replay Gain
Replay Gain (from the English: Playback gain) is a standard that describes how digital audio files can be raised to a common perceived volume without touching the data stored in the file actual audio data. The proposal for this standard was published on 12 July 2001 by David Robinson.
Basics
Are songs from different albums, particularly albums different production date, heard in succession, falls on a sometimes very different perception of loudness. The reason for this may be desired by the producers in the context of an album volume of each song. In most cases, however, the reason in different mastering various albums, and especially the amended over the years it sought " target volume". ( Was For background information see Article loudness. )
The volume -to-peak value, which is sometimes only a few milliseconds long, has only very little influence on the perceived loudness, but is important for the modulation of the overall piece. Traditionally, the fitting is carried out by changing the volume setting. With the availability of digital media and the easy way to create own songs ( Playlists), or the possibility of an album overarching Shuffle, the desire occurs after an automated volume normalization in the foreground.
Although the term was written in the original publication as ReplayGain, the spelling ReplayGain or ReplayGain is used increasingly.
Technology
It is used in two stages: First, once the required loudness information can be determined from the audio data and stored as meta- information together with these. The volume is then adjusted automatically every time you have this information.
First, the relevant files are fully decoded and analyzed. Here (via RMS ) is calculated, a value that should come close to the perceived average volume, as well as recognized the actual peak. This is written as a correction value which brings the difference between the detected perceived average volume and a uniform level of 89 dB specified as additional meta- information in the file - the rest of the file remains untouched.
Only when playing can now be decoded a program, provided it supports the standard, read these values and using at the moment of decoding to correct the actual audio signal.
In order not to leave a single piece of music fall out of the overall concept of an album, the average volume of this album can be calculated as a whole and stored in the audio file. This is used when playing this correction value, relative volume differences between the individual pieces of the album remain the ( unwanted) received.
Since the adjustment takes place when decoding, so it is just a tagging, the rest of the file remains untouched. The changes can thus be easily removed from a non-compatible decoding program they are ignored. The correction is done ideally before lossy compressed files are quantized into the desired final sample depth, so that if the full dynamic range provided by the respective final sample depth, can be used.
This allows Replay Gain compatible audio players, to compensate for the existing differences and play back these files each with about the same average ( perceived) volume. This avoids that manually each time the volume needs to be adjusted, if mastered tracks are played sequentially on different levels. ( This adaptation is not to be confused with the usual modulation, in which instead of the average perceived loudness, the peak level of the individual pieces are brought to a uniform value. )
Although the Replay Gain standard speaks of an 8 -byte field in the header of the file which should be the same for all audio formats, but many formats, such as Vorbis or FLAC, have a custom tag for this information. For MP3s use programs like foobar2000 the way to write ID3v2 tags of type TXXX in the file. The ID3v2 standard provides for some time before a " RVA " field ( Relative Volume Adjustment), which can be used for replay -gain purposes.
Alternatives
Change audio data, recoding
When attaching metadata is not desired or not possible ( for example, in the absence of support from decoder or burning programs ), the output audio data can be changed to bring the perceived volume to the specified unit height as an alternative. This is not only very expensive, but connected by the arithmetic operations with certain sonic losses ( increased noise, increased distortion, at least for 16 bit or less). In volume reduction also the transmitted dynamic range is reduced. In contrast, an increase in volume is not always without intervention in the dynamics course possible ( generation loss ). However, in some encoding types, a scaling factor (reversible ) can be changed, but not in any fine steps.
MP3, AAC and Global Gain
The program MP3Gain can make for MP3 files to lossless and reversible way (but only one - in practice usually sufficient - accuracy of 1.5 dB ). To this end, the global gain fields of the individual frames that define the overall level of each MP3 frame, manipulated. The operation is performed directly on the MP3 structure. It is reversible in most cases. Since no recoding takes place, there are no generation loss. In addition, the file is optional add a tag, which estimates the correction made; by means of which the operation can be undone later if necessary.
The same is true with AACgain for Advanced Audio Coding, and with VorbisGain for Ogg Vorbis files.